The die-die stacking package technology is making rapid development in recent years. The technology is a developing trend of integrated circuit advanced high-density packaging, which is an advanced die assembly technology for achieving more reliable, high performance and higher-density circuits.
FIG. 1 illustrates an existing die-die stacking structure 100. The die-die stacking structure includes dies 101, 102 and 103. Die 103 is disposed below dies 101 and 102. A plurality of fine pitch (e.g. 45 μm) bumps 104 are designed for the connection of dies 101, 102 to die 103. A plurality of fine pitch bumps 105 are designed for the connection of die 103 to substrate 107. To strengthen the stability of the bumps 104, 105 and dies 101, 102 and 103, underfill 106 is applied to fully fill the space between die 101 and die 103, die 102 and die 103, and die 103 and substrate 107 and extend beyond the outer edges and bottom edges of dies 101, 102 and 103 to form a fillet 108.
However, this kind of die-die stacking structure has some shortcomings as follows. First, fully filling the space between dies 101, 102 and die 103, and die 103 and substrate 107 can induce void in the underfill 106 more easily. Also, it is easier to produce die-underfill delamination or popcorn failure induced by moisture and thermal stress in the fully filled package. In addition, the formed underfill fillets 108 surrounding the outer side of dies 101, 102 and 103 may more easily cause dies to crack due to higher underfill fillet height and stain the active area of dies. Furthermore, it can lead to die or component layout space utilization waste due to reservation of die-die/component space for underfill dispensing, fillet and bleeding. Moreover, it needs a long underfill dispensing process cycle time caused by slow underfill due to fine bump pitch design or smaller die gap between die 101 and die 103, die 102 and die 103.
FIG. 2 illustrates another existing die-die stacking structure 200, in which the upper die 201 and the lower die 202 are connected through multiple bumps 203. Underfill 204 is also fully filled between die 201 and die 202 to protect the bumps 203. Obviously, the die-die stacking structure 200 in FIG. 2 also has the above shortcomings
FIG. 3 illustrates another existing die-die stacking structure 300, in which the upper die 301 and the lower die 302 are connected through multiple bumps 303. There is no fully filled underfill between the upper die and the lower die. The bumps 303 are protected by protect type flux 304. However, the flux 304 covering the bumps is insufficient and does not form a solid structure to absorb and resist thermal stress induced by die-die warpage during reflow or thermal cycle test. Therefore, the bump protection for fatigue/stress crack failure in structure 300 is weak due to insufficient material cover the bump and material protection discontinuity.
Therefore, there is a need to provide an improved die-die stacking structure and method that overcome the shortcomings in the existing die-die stacking structures.